It may desirable to run at least some low-pressure exhaust gas recirculation (LP-EGR) during an engine and catalyst warm-up phase to increase heat flux to the aftertreatment devices and to reduce surge while running enough EGR during warm-up to control NOx. However, EGR coolers can condense water out, especially before engine coolant temperature warms to its operating temperature, (e.g., when engine coolant is lower than its warmed-up operating temperature). This can be problematic in any EGR system, but water droplets formed in a low-pressure EGR circuit can particularly degrade an aluminum compressor wheel operating at high speed. One solution is to only use low-pressure EGR during selected coolant temperature ranges; however, this will limit the amount of EGR one can use due to intake manifold temperature and surge limitations.
One method for reducing condensation is disclosed in U.S. Pat. No. 7,469,691. In the cited reference, EGR is routed through a bypass around an EGR cooler to an intake passage of the engine. The EGR is then mixed with intake air in the intake passage before mixing with cooled EGR. However, with the addition of cooled EGR just before the mixture enters a compressor of a turbocharger, a possibility of condensate formation remains in the gases entering the compressor.
The inventors herein have recognized the above issues and have devised an approach to at least partially address them. In one example, a method for an exhaust gas recirculation (EGR) system including an EGR cooler coupled to a turbocharged engine in a vehicle is disclosed. The method comprises, under selected operating conditions, controlling a mixture temperature of cooled and uncooled EGR at a mixing location upstream of an intake passage inlet by routing at least a portion of exhaust gas through a bypass around the EGR cooler.
For example, LP-EGR may be desired under cold start conditions in which the coolant temperature is below a threshold temperature. An EGR cooler bypass valve may be adjusted such that at least a portion of the EGR is routed around the EGR cooler and thus remains uncooled. As such, the temperature of the mixture of cooled and uncooled EGR, and thus the temperature of EGR entering the intake passage, may be controlled to be above a threshold temperature. In this manner, formation of condensate may be reduced, thus reducing a chance of compressor degradation due to condensate formation in the LP-EGR system during a cold engine start.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.